Yes, but I really wouldn't recommend using Kylix any more. It is just too out of date these days (compiler and IDE), and CLX uses a terribly old Qt (that just looks crap - doesn't even support anti-aliased text).

after my app exits, other apps don't see this text. Clipboard empty. Why?

It is standard X11 behaviour, and is actually more efficient than Windows clipboard. Why, because on Windows, if you copy something to the clipboard you instantly get a copy, and thus it uses memory (RAM) to store that copy. On X11 when you copy something to the clipboard, it simply notifies the windowing system of the window that has content available, but no copy actually exists in RAM yet. Once once a target applications requests that content (via a Paste action), does it query the source window/application for the contents. This explains why you clipboard is empty, because that source application has already terminated.

Now some newer Window Managers in X11 support clipboard managers that can go against the X11 design. What these clipboard managers do, is become the "source" of the clipboard content, instead of the original source application. This clipboard managers keep running while the window manager runs, so you never get the "out of scope" situation.

If you want to see the technical details in how this is implemented, so applications (actually the GUI framework) can take advantage of such clipboard managers, take a look at how it was implemented in fpGUI Toolkit.

Some desktop environments or window managers include support for a clipboard manager. This clipboard manager can make the content of the clipboard persistent even after the source application closed down.

There is the small complication that keeping data on the clipboard after exiting a process is technically a memory leak.That's also the reason that X11 developers won't fix it. It is unsafe behavior. A clipboard manager merely takes ownership of the data but clipboards are inherently not secure unless the manager encrypts and decrypts the data.

This is a known Windows design flaw, although even on Windows the clipboard data is only accessible locally after process termination.